The theme of both the Sonnets is similar, both the poets wish to immortalize the names of their beloved in their sonnets.
The identity of Shakespeare's lover, the person to whom his sonnet is addressed is yet to be established with certainty. Spenser's sonnet is addressed to his wife Elizabeth Boyle whom he married on June 11th 1594.
Shakespeare's sonnet is a direct address to his lover, whereas Spenser's sonnet is in the form of a conversation with his wife.
Shakespeare compares his lover with only one object,"summer's day" and lists out the negative aspects of summer season and concludes that his lover who is a personification of all the positive qualities of "eternal summer shall not fade." Spenser, on the other hand, does not compare Elizabeth Boyle to any object. After the waves have washed away her name, he tells her how he intends to immortalize her and their mutual love in his verse. Hence, Shakespeare's sonnet is denser because it is packed with more images-word pictures which stimulate our imagination.
Shakespeare's sonnet has the rhyme scheme: ababcdcdefef gg whereas Spenser's sonnet has the rhyme scheme: ababbcbccdcd ee. The number of rhymes in Spenser's sonnet is lesser than the number or rhymes in Shakespeare's sonnet and unlike the quatrains in Shakespeare's the two quatrains in Spenser's sonnet are 'linked'- bb and cc. Spenser's sonnet is thus more tightly constructed.